Conventionally, exhaust purification has been tried by an exhaust purifying catalyst incorporated in an exhaust pipe. Known as such exhaust purifying catalyst is a NOx-occlusion reduction catalyst having a feature of oxidizing NOx in exhaust gas for temporary occlusion in the form of nitrate salt when exhaust/fuel ratio is lean, and decomposing and discharging NOx through intervention of unburned HC, CO and the like for reduction and purification when O2 concentration in the exhaust gas is lowered.
Known as such NOx-occlusion reduction catalyst having the above-mentioned feature is, for example, an alumina catalyst carrying platinum and barium or platinum and potassium.
Since no further NOx can be occluded once an occluded NOx amount increases into saturation in the NOx-occlusion reduction catalyst, it is periodically required to lower the O2 concentration in the exhaust gas flowing into the reduction catalyst to decompose and discharge NOx.
In application to, for example, a gasoline engine, lowering the operational air/fuel ratio in the engine (operating the engine with rich air/fuel ratio) can lower the O2 concentration and increase the reduction components such as unburned HC and CO in the exhaust gas for facilitation of decomposition and discharge of NOx. However, in use of a NOx-occlusion reduction catalyst in an exhaust emission control device for a diesel engine, it is difficult to operate the engine with rich air/fuel ratio.
This results in necessity of newly arranging means for adding fuel (HC) into the exhaust gas upstream of the NOx-occlusion reduction catalyst so as to react the added fuel as reducing agent with O2 on the reduction catalyst to thereby actively lower the O2 concentration in the exhaust gas to regenerate the reduction catalyst.    [Reference 1] JP 2000-356127A